Borehole collapse, circulation loss, and oil and gas formation protection are three major technical difficulties that constrain successful implementation of drilling engineering. For formations where beddings and fractured are developed, especially mud shale formations, to prevent borehole collapse and circulation loss and reduce damage to the oil and gas reservoir, the drilling fluid must have high plugging performance and can effective plug the beddings and crevices, so that the drilling fluid and its filtrate can't infiltrate into the formation, in order to improve formation bearing capacity, stabilize well bore, prevent circulation loss, and protect gas and oil reservoir. To that end, researches have been made on methods for evaluating plugging agents used in drilling fluids and relevant influencing factors.
Since the twentieth century, shale gas exploration and development has been carried out in China, and dozens of shale gas wells have been drilled. To prevent wall sloughing of horizontal shale gas wells, oil-based drilling fluids with a strong inhibition property are usually used, but the well-bore instability problem can't be solved with oil-based drilling fluids or synthetic drilling fluids satisfactorily. There are corresponding core ideals for solving the well-bore instability problem of different shale wells in foreign countries: for Haynesville shale, a solution to the shale dispersion problem should be considered mainly; for Fayetteville shale, shale cleavage incurred by crevices should be considered mainly; for Barnett shale, inhibition of hydration should be considered mainly.
For drilling fluids that have a strong inhibition property, one of the main reasons for well-bore instability is the transfer of hydraulic pressure through micro-crevices; therefore, the plugging property of the drilling fluids against micro-crevices must be improved. Micro-crevices are usually in size between nanometer and micrometer; conventional plugging agents are too large in size to provide a satisfactory plugging effect; therefore, plugging with nanometer materials is a good method. However, if nanometer particles are added into a drilling fluid, the solid content and sub-micrometer particle content in the drilling fluid will be increased inevitably, causing increased opportunity of damages to the oil and gas reservoir by fine particles; in addition, since the materials added into the drilling fluid have high surface activity, they will absorb a part of drilling fluid adhesives, causing decreased effective contents of the drilling fluid adhesives; moreover, the material may agglomerate easily, and the agglomerated particles are much larger in size and are difficult to be dispersed to nanometer level merely by agitation; consequently, the agglomerated particles lose the properties of nanometer particles.
Though some breakthroughs have been made in nanometer plugging agents, there are still many problems: firstly, as calculated by Al-Bazali with a capillary pressure formula in 2005, the pore throat sizes of mud shale are distributed between 10 nm and 30 nm; according to the one-third plugging theory, the particle diameter of the plugging material should be approximately equal to one third of the pore size, which means the plugging material can attain a good mud shale plugging effect only if the particle diameter of the plugging material is between 3 nm and 10 nm; however, most nanometer plugging agents developed domestically in China have particle diameter greater than 50 nm, and can't achieve effective plugging. Secondly, the material may agglomerate easily, and the agglomerated particles are much larger in size and are difficult to be dispersed to nanometer level merely by agitation.